Heat the agave, water, and coriander in a small saucepan over low heat until the syrup comes to a gentle simmer. Add the cilantro and lime zest, remove from heat, cover, and let sit on the stove to infuse for an hour. Pour through a fine sieve and keep refrigerated in a sealed container. Optionally, add 1/2 oz of 100-proof (or higher) vodka for longer storage - though note that this technically makes it non-alcoholic.

My favorite usage thus far:

2 oz blanco tequila
1/2 oz Cilontrastic Syrup
1/2 oz lime juice

Shake and strain into an old-fashioned glass over crushed ice.

This works equally well with other robust white spirits, like white rum or pisco.